In some areas of technology there is a need for reading and/or digitizing a time varying signal of which the value changes over many orders of magnitudes. In some cases the signal value changes can occur fast.
An example of such an area of technology involves measuring transient electromagnetic responses following excitations of an earth formation. U.S. patent applications published under numbers 2005/0092487, 2005/0093546, 2006/0038571, each incorporated herein by reference, describe such transient electromagnetic (EM) methods for locating an anomaly in a subterranean earth formation, and in particular for finding the direction and distance to a resistive or conductive anomaly in a formation surrounding a borehole, or ahead of the borehole, in drilling applications.
In these methods, typically a tool comprising a transmitter antenna, a receiver antenna, and a structural support, is lowered into a borehole in the earth formation. A transient response signal, comprising an induction voltage in the receiver antenna resulting from a sudden change in the current that is passed through the transmitter antenna, is measured. The referenced US patent applications show that response signals can decay from microvolts to nanovolts in microsecond times. This triggers a demand for high-speed high-dynamic range detection- and data acquisition circuitry.
Several methods and apparatus are known for improving the dynamic range in analogue to digital converters.
In some commercially available hand-held multi-meters, such as Volt-Ohm meters, gain switching circuitry is provided to switch between gain settings of a signal amplifier.
Roos et al, in a U.S. Pat. Application published under number 2005/0083120, disclose data signal amplification and processing switching circuitry with multiple signal gains for increasing dynamic signal range for X-ray imaging pixel value signals. The multiple signal gains are obtained by sequentially switching in more feedback capacity over a differential amplifier.
Such switching circuitry may introduce relatively long time lags through periods of gain switching, and time is also lost when the amplifier is set at a sub-optimal gain. Both result in loss of data during such periods.